Symptoms:

cc -DHAVE_CONFIG_H -I. -I../../..  -O2 -fno-strict-aliasing -pipe -g -Wall -finline-functions -O2 -I/usr/local/include -DHAVE
_SILCDEFS_H -I. -I../../.. -I/usr/ports/net/silc-client/work/silc-client-1.1.1 -I/usr/ports/net/silc-client/work/silc-client-
1.1.1/lib/contrib -I/usr/ports/net/silc-client/work/silc-client-1.1.1/lib/silccore -I/usr/ports/net/silc-client/work/silc-cli
ent-1.1.1/lib/silccrypt -I/usr/ports/net/silc-client/work/silc-client-1.1.1/lib/silcmath -I/usr/ports/net/silc-client/work/si
lc-client-1.1.1/lib/silcutil -I/usr/ports/net/silc-client/work/silc-client-1.1.1/lib/silcapputil -I/usr/ports/net/silc-client
/work/silc-client-1.1.1/lib/silcske -I/usr/ports/net/silc-client/work/silc-client-1.1.1/lib/silcsftp -I/usr/ports/net/silc-cl
ient/work/silc-client-1.1.1/lib/silcclient -I/usr/ports/net/silc-client/work/silc-client-1.1.1/lib/silcasn1 -I/usr/ports/net/
silc-client/work/silc-client-1.1.1/lib/silcskr -I/usr/ports/net/silc-client/work/silc-client-1.1.1/lib/silcvcard -I/usr/ports
/net/silc-client/work/silc-client-1.1.1/includes -I/usr/ports/net/silc-client/work/silc-client-1.1.1/doc -I../../../src -I../
./../src/core/ -I/usr/local/include/glib-2.0 -I/usr/local/lib/glib-2.0/include   -DHELPDIR=\""/usr/local/share/silc/help"\"
-DTHEMESDIR=\""/usr/local/etc"\"    -O2 -fno-strict-aliasing -pipe  -Wall -MT fe-common-core.o -MD -MP -MF .deps/fe-common-co
re.Tpo -c -o fe-common-core.o fe-common-core.c
In file included from fe-common-core.c:35:
/usr/local/include/recode.h:65: warning: parameter names (without types) in function declaration
/usr/local/include/recode.h:66: error: syntax error before "recode_delete_outer"
/usr/local/include/recode.h:66: warning: type defaults to `int' in declaration of `recode_delete_outer'
/usr/local/include/recode.h:66: warning: data definition has no type or storage class
/usr/local/include/recode.h:68: error: syntax error before "list_all_symbols"
/usr/local/include/recode.h:68: warning: type defaults to `int' in declaration of `list_all_symbols'
/usr/local/include/recode.h:68: warning: data definition has no type or storage class
/usr/local/include/recode.h:69: error: syntax error before "list_concise_charset"
/usr/local/include/recode.h:69: warning: type defaults to `int' in declaration of `list_concise_charset'
/usr/local/include/recode.h:69: warning: data definition has no type or storage class
/usr/local/include/recode.h:71: error: syntax error before "list_full_charset"
/usr/local/include/recode.h:71: warning: type defaults to `int' in declaration of `list_full_charset'
/usr/local/include/recode.h:71: warning: data definition has no type or storage class
/usr/local/include/recode.h:78: error: syntax error before "recode_delete_request"
/usr/local/include/recode.h:78: warning: type defaults to `int' in declaration of `recode_delete_request'
/usr/local/include/recode.h:78: warning: data definition has no type or storage class
/usr/local/include/recode.h:79: error: syntax error before "recode_scan_request"
/usr/local/include/recode.h:79: warning: type defaults to `int' in declaration of `recode_scan_request'
/usr/local/include/recode.h:79: warning: data definition has no type or storage class
/usr/local/include/recode.h:81: error: syntax error before "recode_format_table"
/usr/local/include/recode.h:81: warning: type defaults to `int' in declaration of `recode_format_table'
/usr/local/include/recode.h:81: warning: data definition has no type or storage class
/usr/local/include/recode.h:87: error: syntax error before "recode_string_to_buffer"
/usr/local/include/recode.h:87: warning: type defaults to `int' in declaration of `recode_string_to_buffer'
/usr/local/include/recode.h:87: warning: data definition has no type or storage class
/usr/local/include/recode.h:90: error: syntax error before "recode_string_to_file"
/usr/local/include/recode.h:90: warning: type defaults to `int' in declaration of `recode_string_to_file'
/usr/local/include/recode.h:90: warning: data definition has no type or storage class
/usr/local/include/recode.h:93: error: syntax error before "recode_buffer_to_buffer"
/usr/local/include/recode.h:93: warning: type defaults to `int' in declaration of `recode_buffer_to_buffer'
/usr/local/include/recode.h:93: warning: data definition has no type or storage class
/usr/local/include/recode.h:96: error: syntax error before "recode_buffer_to_file"
/usr/local/include/recode.h:96: warning: type defaults to `int' in declaration of `recode_buffer_to_file'
/usr/local/include/recode.h:96: warning: data definition has no type or storage class
/usr/local/include/recode.h:99: error: syntax error before "recode_file_to_buffer"
/usr/local/include/recode.h:99: warning: type defaults to `int' in declaration of `recode_file_to_buffer'
/usr/local/include/recode.h:99: warning: data definition has no type or storage class
/usr/local/include/recode.h:102: error: syntax error before "recode_file_to_file"
/usr/local/include/recode.h:102: warning: type defaults to `int' in declaration of `recode_file_to_file'
/usr/local/include/recode.h:102: warning: data definition has no type or storage class
/usr/local/include/recode.h:111: error: syntax error before "recode_delete_task"
/usr/local/include/recode.h:111: warning: type defaults to `int' in declaration of `recode_delete_task'
/usr/local/include/recode.h:111: warning: data definition has no type or storage class
/usr/local/include/recode.h:112: error: syntax error before "recode_perform_task"
/usr/local/include/recode.h:112: warning: type defaults to `int' in declaration of `recode_perform_task'
/usr/local/include/recode.h:112: warning: data definition has no type or storage class
gmake[7]: *** [fe-common-core.o] Error 1
gmake[7]: Leaving directory `/usr/ports/net/silc-client/work/silc-client-1.1.1/apps/irssi/src/fe-common/core'
gmake[6]: *** [all-recursive] Error 1
gmake[6]: Leaving directory `/usr/ports/net/silc-client/work/silc-client-1.1.1/apps/irssi/src/fe-common'
gmake[5]: *** [all-recursive] Error 1
gmake[5]: Leaving directory `/usr/ports/net/silc-client/work/silc-client-1.1.1/apps/irssi/src'
gmake[4]: *** [all-recursive] Error 1
gmake[4]: Leaving directory `/usr/ports/net/silc-client/work/silc-client-1.1.1/apps/irssi'
gmake[3]: *** [all] Error 2
gmake[3]: Leaving directory `/usr/ports/net/silc-client/work/silc-client-1.1.1/apps/irssi'
gmake[2]: *** [all-recursive] Error 1
gmake[2]: Leaving directory `/usr/ports/net/silc-client/work/silc-client-1.1.1/apps'
gmake[1]: *** [all-recursive] Error 1
gmake[1]: Leaving directory `/usr/ports/net/silc-client/work/silc-client-1.1.1'
gmake: *** [all] Error 2
*** Error code 2

Stop in /usr/ports/net/silc-client.
** Command failed [exit code 1]: /usr/bin/script -qa /tmp/portupgrade.55139.0 env UPGRADE_TOOL=portupgrade UPGRADE_PORT=silc-client-1.0.3 UPGRADE_PORT_VER=1.0.3 make
** Fix the problem and try again.

Piece of preprocessed source of the .c file:

[...]
# 65 "/usr/local/include/recode.h"
RECODE_OUTER recode_new_outer (bool);
[...]

It seems like bool is not being defined. It seems that the recode and header require C99 to compile but the silc port does not compile with -std=c99 switch.

Fix: 

Quick fix/hack could be that all of the files of the silc port that include recode.h also include stdbool.h.

IMHO proper fix should be to fix the recode port to either include stdbool.h or that it does not require it at all.